If your skates are too big, your blades are in the wrong place and you learn really bad habits that take years to learn once they become habit.
Don't advise someone to order skates too big.
Yes and no.
I would claim that for most people, blades are most likely put in the wrong place by a merely average skill skate tech anyway, unless you get properly measured fully customized skates, with an outsole positioned and sized to match the skater's particular anatomy.
I did of course assume that someone willing to take their own measurements is going to take care to mount the blade right for their feet, and not pay any attention to low end skate tech mounting advice. I'm also assuming that when she cuts the insoles, she will do it right. The link I sent her has fairly detailed advice on that.
As near as I can tell, for most people, figure skate boots are ideally designed so as to make the heel especially snug at the back, and to place the part of the footbed+insole that bends upwards match the ball of the foot. (I don't quite understand all the reasons for the upwards bend in the footbed+insole shape, but it does help prevent the foot from sliding forwards, especially if the boot isn't fully customized in shape.) They should also be fairly snug almost everywhere, 3-dimensionally, except at the front and sides of the toes, where most people need a little extra space to keep the feet healthy, and they have a little space ahead and in front of the ankle, so they can freely point and flex the ankle. And you might need to vary the insole shape and stiffness at various points, or pay someone really good to create custom orthotics.
Figure skate blades, in turn, are designed to be mounted so as to put a particular part of the blade (roughly where the sweet spot is) underneath the ball of the foot. They are designed to be sized so that the tail of the blade interacts well with the back of the foot - i.e., you don't keep crossing your tail and tripping, but neither does the back of the blade drag when you skate. And you want the toe pick to interact well with the front of your foot - though that is too many parameters to match well, given that there is only one variable in standard blade sizing - length. (Though other variables can to some extent be chosen by varying blade model.) (High end skate techs can reshape the blade to move the sweet spot to make things work better, but the o.p. probably doesn't have one, since she is doing her own measurements.)
This is all quite complicated and is beyond the skill level of the merely average skate tech. You need very complicated measurements to get it all right, and to know what foot shape assumptions the specific boot maker uses.
So, typical advice given to low end skate techs, based on the boot makers conception of the typical foot (which incidentally varies, brand to brand) involves very simple measurements to pick the size. Low end skate techs might size boots either by the total length, or by the total width. And they are told to always place the front of the mounting plate wherever the front of the outsole is - which is again based on the boot maker's conception of typical feet. Low end skate techs then choose the blade length so the back of the mounting plate roughly matches wherever the back of the outsole happens to be. (I think they rarely size the blade to go beyond the back of the outsole - more likely they are told to pick a length that comes as close as possible to matching the back of the outsole, without going past it.)
But everything can go wrong when you design and measure for the "typical foot", rather than a specific foot.
The problem is that the relative position of the everything within the foot varies a lot, person-to-person, depending on things like the relative lengths of your toes compared to your feet, and the way the back of the heel relates to the back of the upper foot. What is more, your left foot and right foot are probably sized and proportioned differently. I.E., most people don't have "typical" feet. And that doesn't even begin to cover the overall side-to-side and forward-to-back tilt of the bottom of your feet, the amount of arch (at each of the several major arches in your feet) or the degree to which your feet collapse asymmetrically when you put weight on them - which is particularly important to figure skating, because once you set an edge for a lobe, you are often "supposed to" hold that degree of edge, so the curvature of the lobe doesn't vary. (There are other complications that might matter too, involving the rest of the body, like how your knees and hips move, but I don't understand how that all works.)
So yes - ordering a little large will probably position - and size - the blades wrong. So, probably will ordering the boots snug. The best thing is for her to figure out where to mount blades to match her feet after she gets the boots. If you leave a little space, you have room to play, if you are willing to play and experiment. Hopefully, she already has blades whose length and sweet spot positions works well with her feet and body.