does this make sense?

[crisgal]

"She has passed on a wealth of knowledge to Thomas, who shares her enthusiasm for caring about others and the world around him."

or should the final word be 'her' or 'them'?

[Count-A-Strong]

Deffo.

[albaqwerty]

the world around him (Thomas)

[Quizmonster]

Why not use 'them', which removes any question of gender?

[charliesim]

I'd also go for "around him" because Thomas shares her enthusiasm for caring about others and the world around him

[Ankou]

its talking about a shared passion. i would say 'them'

[Quizmonster]

The natural assumption here is that the 'she' is Thomas's mother or some other loving female relative and that he is a child. However, they could equally both be adults, say professor and student.
If he is a child, she might actually be a nanny or tutoress who can't stand the little blighter...that is, she may not care a good goldarn for the world about HIM but does for the world about HER. Nevertheless, as a dutiful employee, she has taught him many things and - as a result - he has acquired many of her views, albeit he is applying them in a different way.
If you use 'them', as I suggested earlier, you cover both possibilities; namely, that these two people may occupy what amounts to separate worlds or the same world. (Yes, I know there is only one actual world in a physical sense but, in another way, we each occupy our very own world.)

Stick with 'them'.

[Old_Geezer]

I'd use 'him', but I think 'them' would work even if it does change the meaning slightly. I suppose 'her' would to but there again the meaning changes again. It's always the same world, but ....

Pick one. I'd go with 'him'.

[crisgal]

well, i'm glad it wasn't just me who doubted myself!
i've gone with them.

[JonnyBoy12]

There are 2 schools of thought on this one. I would go with the word "them" personally.

[factor-fiction]

How about 'us'
(Wasn't that a song title by Champagne?)

[AlBags]

Thomas is the subject .. so it's 'him'

[MarkRae]

Almost, Factor, but it was "Champaign" as in the county in Illinois rather than the fizzy drink...

[Quizmonster]

Good choice, Crisgal!

[chakka35]

Am I really alone in preferring 'her'? He is sharing her enthusiasm, not the other way round, and her enthusiasm is for the world around her.

[jno]

yes, but his own enthusiasm is (probably) for the world around him, rather than the world around her. "Them" seems sensible as it would avoid the ambiguity.

[Quizmonster]

My point exactly, J, in my second response above. Cheers

[Prudie]

I'd go with 'her'. The object Thomas shares is "her enthusiasm for caring about others and the world around her".
Anyway him seemed fine on first reading.

[chakka35]

Yes,OK, you can justify either 'her' or 'them', but I prefer 'her' as being syntactically purer while 'them' is merely safer.

[Quizmonster]

I don't agree with "merely" safer. The plain fact is that there are two potential ambiguities in the original sentence...
a) her enthusiasm/his enthusiasm
b) her world/his world.
We know for sure that the enthusiasm is shared, so they/them is clearly OK, but we are unsure whether their 'worlds' are identical. Accordingly, maybe the best solution would be a complete rewrite, but if - as I imagine Crisgal wanted - we are looking for a minimal alteration, then 'them' is simply the best (at the risk of sounding like Diana Ross or whoever!)

A possible alternative...
She has passed on a wealth of knowledge to Thomas. As a result, he cares as much about others as she does and has the same enthusiasm for his world as she has for hers.

Clumsy, obviously, so that's why I still believe 'them' is the neatest solution.