It really depends on what your daughter's priorities are and what future pattern of flying she s likely to do apart from these flights.
If she is likely to remain just an occasional flyer, the key issues will be: 1) how many miles you get, 2) how easy it will be to redeem and 3) whether the miles expire or not.
Regarding 1), rules vary from one FFP to another as to which fares are eligible and which amounts of miles you get. As to 2), as a rough rule of thumb, it is usually easier to redeem on the airline whose FFP you are a member of.
As to 3), most FFPs let you keep your miles forever as long as there is some flight activity to keep the account alive every 3 years.

If there is going to be more flying, your daughter might think about getting status and, here, Air Canada and BMI would be definitely worth considering as they have substantially lower thresholds for elite qualification.