Pay close attention to the body language of your eevee, anonymous. Mai, as an Alolan raichu, is half-psychic and highly empathic as a result. Thus, it’s likely she’s picking up on your eevee’s anxiety about evolving (at all or into jolteon specifically) and trying to stop you from forcing your eevee to evolve against his will. Communicate with your eevee and make it absolutely clear to him that evolution is his choice, including and especially when it comes to the topic of what he wishes to evolve into. If he seems hesitant at the idea of becoming a jolteon, ask him if he would prefer a different form or if he would wish to stay an eevee. Be sure to go over all of his options carefully so he can make the decision on his own. If you absolutely must have a jolteon, you can try convincing him, but do so in a way that makes it clear you have his interests at heart.
If he actually does want to evolve into a jolteon, then the other possible issue at play here is Mai’s worry that you’re getting another electric-type to replace her on the field. If this is the case, then be sure to reassure her that she will always be valuable to your team and that you by no means intend on retiring her.
Of course, there is also the possibility that neither of these are Mai’s concerns, at which point you’ll need to communicate with Mai as much as possible to figure out what it is that’s troubling her. Take her aside, sit her down, and ask her about her concerns. Keep reassuring her that you only want what’s best for your entire team, but you’re willing to be perfectly flexible to meet their needs.
Best of luck, anonymous!
The Pokédex According to Bill 