Yes, it is possible to have too many. And I've got too many.... way too many
The question(s) really should be; "How can my friend bring himself / herself to sell some of his / her basses?"
and "What therapy is available to help him / her do so?"
If you`ve a use for them all, why not. It`s our hobby/livelihood, why shouldn`t we have the instruments we want. Sure, a rationale is only having what you need, but if you can afford more, nowt wrong imo.