Breastmilk got asked to thrown away

I think your MIL spoke up out of concern for the baby, please don't take it personally. Personally I won't be comfortable too using a towel, but in my case it's because my towels are quite exposed in the kitchen and had been soaking up oil fumes over time. You would know your situation best, but do take her advice under consideration. Can consider this article too, towel dying is indeed not the most recommended method: https://mothersneed.com/how-to-dry-baby-bottles/