I use a Mac as my primary work computer and I use Forms heavily. I haven't noticed any issues or shortcomings that are Mac-specific.

Perhaps the issue is caused by the way the user's email client on Mac treats embedded links. Hard to say though without knowing the exact symptoms. Ask them if they get any error messages.

Forms is web based. It's based off internet standards. If something doesnt work, it's likely due to the browser being out of date or a rare occurrence/change in how 1 browser renders something versus another.

I would recommend checking to ensure your machine is within your network so that it can reach the server itself. Then ensure that Safari is up to date.

In Parallels it may be set up to instantly connect via VPN to your network and that would be a reason it would work on windows and not mac. You will need to overcome that hurdle on the mac side if that is the case.

What error message do they get? Can you reproduce the issue and take a screenshot then post it here?

Hell Trevor,

Another thing to look into along with what everyone else has posted, is the exact version of Forms including the build. This can be found from the top right drop down > About. Providing this information would be helpful to see if perhaps we can reproduce the issue. 

Regards