Well that's very true unfortunately. But the flights on AA (I presume) should allow free seat reservations on them via aa.com, so 2 of the legs should be OK, with a bit of careful co-ordination. For the last one remaining, namely the return to the UK, maybe another upgrade for her? As LTN_Phobia pointed out it can't be that much of an issue being sat together if that factor is rubbed out so long as she's in another cabin altogether.