a rant and then a question

I hate my mother in law. I used to just find her annoying but she's now messed up 2.5 months worth of potty training. my son began potty training himself a couple weeks before he turned 2. he just went one day. so I started really potty training him. in 2.5 months he was staying dry most of the day and some days, not using pull ups but just his training underwear. yay! he was doing so good! so this weekend my mother in law took him so my husband and I could celebrate our anniversary. i told her about his potty training and how to go about it. she dropped him back off Saturday night and I asked how potty training went and she said she didn't do any because they were constantly busy. I told her he was fine using public potties and she argues that she didn't want to expose him to those germs. like wtf! picnic tables have more germs than public toilets! now it's Friday and I haven't been able to get my son to even sit on the potty. he screams if he realizes I'm trying to get him to go. so that's the rant. now my question is how do I get my son to continue potty training? please