I had to pump in public. The brownies had a rare chance for an overseas residential trip for several days. DS2 was 1. The other leader a few months postnatal. So I went. I thought when signing up many months in advance, that DS would be most likely to be feeding morning and bedtime like his sibling. He was a total milk monster on my non-working days and still fed frequently in the day. I could last a working day, but on day 2-3 of the trip, the engorgement peaked. So in order to prevent yet another bout of mastitis (the previous one causing painful circulation problems), pumping regularly to relieve the engorgment was essential. That meant my pump was up my Guiding uniform on buses, on trams, around museums, walking around the streets. The children were vaguely curious at first, and I told them that I had to take away the spare milk that my baby would normally drink and they completely accepted it and moved on.
Disgust for something like BFing/ pumping is learned behaviour.
The problem is we don't see it to normalise it. We notice few BFers because it is more subtle than people expect so many people continue to be "digusted" that a mother makes her milk to feed her baby. Being in a position to need to pump in public is fairly unusual so isn't normalised. Then people who are critical about BFing often create fantasies about women getting some kind of pleasure for themselves about BFing (and they don't mean the sheer relief of not feeling like an iron like breast is about to explode) which adds to their "disgust" and moral outrage.
Pumping is very important as a source of breast milk to some babies, and at giving independence to mums. It's not dirty so shouldn't be hidden away in toilets. From OP's description, she's trying to be subtle anyway, and has been invited to stay where she was by her friends so was completely unreasonable.
I hope OP's friend doesn't enounter obsticles from the MiL when it's her turn to have and feed babies.