some of these are already mentioned somewhat..
1) although u say she will rent - look into getting boot heaters. These are rechargable battery packs with insoles where the toe / ball of the foot have heating wires. They work. U dial up the heat. More heat drains more battery.
2) as mentoned keep the body core warm - but remember the greatest source of heat loss are the exposed areas - the head, face, neck, wrists, ankles....so look after them. Good goves helmet/hat
3) some people say wash ur feet the moring of skiing and dry them -even use an antiperspirant to keep them dry because wet is cold..stop perspiration.
4) boots can't be too tight, cuts off circulation so u gotta work that out....maybe just leave the toebox loose and mid & top buckles tight.
5) use a hi-end sock - not a thick sock. either some kind of merino wool or a synthetic fabric that wicks away moisture or breathes - but most important change socks before skiing and at lunch or half way thru the day....which means carry a spare pair. Air out the boots at lunch time...