Whether to accept co-authorship for conference presentation after making only a small contribution?
My answer is based on experience in the field of applied mathematics.  I'm assuming (as stated in the question) that there is no written publication involved -- just a talk.
Don't worry about claiming unmerited credit
You have made some contribution.  Coauthorship on a conference presentation carries almost zero significance for purposes of evaluation.  It's not something you list on your CV, no matter how &quot;early career&quot; you are.  Usually, if you are not already well-known and the results are not earth-shattering, then nobody will even remember, five minutes after the talk, that you were a co-author.
When to insist on being removed
Obviously, coauthorship of the talk has some significance to you and to the presenter.  Since the presenter has already made it clear that he wishes to include you as coauthor, I would only insist on being taken off if:

The scientific work or the quality of the talk is so bad that you think it will reflect negatively on you; or
Your involvement is so minimal that you couldn't say anything interesting about the project (if asked by someone who attended the talk).

It doesn't sound like either of these conditions apply, so my advice is to let it go.