If the mounting doesn’t lead to fighting, then I think you should just let this play out. By her running away, she’s telling him that she will no let him mount. Eventually he’ll get the message that she wants to be groomed, not mounted!
I think this may actually be something to push through in longer sessions. My Bun Jovi is a mounter, and it didn’t really get better until much longer sessions. In longer sessions, adding a tunnel or a hide with at least 2 exits can help break up chasing and mounting interactions as well.
Some pairs also have some level of mounting in their bond. Bun Jovi will occasionally chase Myra around trying to mount her (especially in spring time), so I have some extra tunnels and hides so she can get a break from him if needed.
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The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.