I think you should tell the owner of the ram ASAP. It'll be far more of a burden if she subsequently has to deal with the problem in her flock as well because she didn't know.
Many years ago I lent a ram to someone and while he had him he bought in a couple of sheep from market. These sheep had scab and gave it to all the sheep they were put in with, including my ram. My ram was then returned just before mine were due to lamb. Nothing was said so all my ewes developed scab and it showed up just as they were lambing, so I couldn't dip them. I had to inject them all with dectomax and just pray that it had killed the scab. It did get rid of it but my some of my ewes were in a pretty bad state as I initially thought it was just lice. When I asked the man who'd borrowed my ramabout it - he said he presumed my animal had brought the scab in so I would have known. Words failed me. But I won't ever lend an animal out again.