It could be simply a dominance issue - having first met her when she was receptive, and she didn't mind him trying to mount her, he now thinks that he can rule the roost !! We went though this with our two, although Barley was far too much the alpha female to let it go anywhere !!
I think you need to let them decide who can mount whom; if she gets fed up of his messing around, she will pretty soon let him know ....
I agree thatr pyometra can replicate the hormonal 'come-on' that a season puts out, but that manifests pretty quickly, and if the Lab had an infection, you (and her owner) would be aware by now !!
Maybe she is just very laid back and doesn't mind his amateurish attentions !!