Russell Group means certain level of research activity plus a medical school.
It depends what the person wants to do eventually.
If they want to go into University work/academic research or something very technical/specialised, then they should go to the University with the best reputation (RAE ratings going back a number of years for consistency) for that particular speciality, with other factors like cost, what the social life is like, how good the library/facilities are factored in as tie-breakers.
If they don't want a research type career, then it's more about course suitability for what they eventually want to end up doing, and what the general perception in that field is of different institutions.
If you look at RAE ratings, there can be some surprises. Some universities with great overall reputations have weaknesses, some lesser lights have great strengths. And sometimes this really means nothing because a particular institution's brand or links are so strong in a particular sector.
To be honest, given how expensive study in the UK is getting, if they are really a high flier, think about study at a US institution too. Might not end up a lot dearer and their international reputations and networks can be much better. It's only really Oxbridge (outwith specialist institutions in the Arts etc) that has any claim to be on a similar footing in this country.
So, in short, I think Russell Group is a less useful shorthand than you might think.