He could always kill her instead. As far as her targets are concerned she's going after anyone involving white knights, something she definitely associates him with. If you're desperate for Diarmuid to be the one killed she can try to kill him.

And, as I said, I don't know if Diarmuid qualifies as someone she could hold back with. The only ones I know for certain would be Archer, Rin, and not!Shirou. However she wouldn't be afraid to go all out on Archer. Diarmuid, close as they are, I don't think she's close enough to be able to hold back despite how much they're working together.