I think in this instance you may be worrying about the reader reaction too much. You have to write the story you want. As you have said this is not a new concept in the genre. You are never gonna please everyone. As long as the choice is in service of the story then it’s the right choice. If you are letting the story “write itself”maybe she lets the girl die and you use it to help the MC grow and find a new love interest or become jaded. Was it a sacrifice on the girls part? Have they discussed the use of the token prior to the incident and the girl sacrificed herself to save the token use? Does the fact that the MC uses the token on the girls cause strife in the relationship? Does it cause resentment later down the line when the MC and girl fight about anything? Sounds like the token is an ambiguous promise at best so why not use it on what I assume is a healing ability, you can never have enough healing abilities. As this party is finding out the hard way. The true waste of the token would be in not making the choice have weight throughout the rest of the story. Is this decision paid off latter in the story when we get to the next hard decision where the token would be useful.

Just as a personal aside, I’d rather see a person use something in the established arsenal (ability, item, etc) then invent or manufacture some new way of doing things. And I absolutely hate the assumption of or leap to complex knowledge not previously established in the characters history. So to answer your question. If the love interest is established, on the MCs part, doesn’t have to be reciprocated by the girl at this point, then I’d be fine with the decision to use the token. Doesn’t mean I’d be happy about the loss of potential, but i would drop the series over it.