Why couldn't it be "with her," though? My students listen to things together all the time ...
It would require another structure in Irish. You éist le someone, even if le generally meana 'to'. When not on mobile, I'll show other structure.
I think part of the problem is that the only translation offered for "léi" is "with her."
i though lei was with her!
It can also mean "to" or "for" in certain circumstances. In this case, you listen le someone, and it translates best to English as "to".