Her usefulness is a bit limited. Waver offers battery charge that can support all servants, while Tamamo is more focused towards supporting Arts and NP damage. With the Special Ascension to skip one of your servants to their final ascension and unlocking all skills, Waver has access to the full 50% battery he offers, while skill levels simply increase the other effects of each skill and reduce the cooldown. With him, your friend could pick any support from their friend list and get out an NP quickly for fast damage.

Tamamo is a good servant in the right teams, she simply won't offer as much to a new player. Waver doesn't care who he is with.

He should take her. No questions asked. Love over meta

Just as Delta said Waver is probably the best choice Gameplay wise.

But if he really wants Tamamo than i'd suggest getting her, since the Game is more Fun using units you like than simply for Meta purposes.

Both of then are in the general Pool so they CAN be acquired anytime, but you'll never have a guarantee to even get them anytime in your lifetime outside of the SSR Tickets.

Gameplay wise: Waver > Tamamo > good servants that they lacking (preferably Enkidu\Ozzy\Altria)...

So, she isn't the best possible choice, but... not the worst or bad one at all.