I actually have used my sisters EBM to top up DS in the early days. Our boys are 3 weeks apart and she has a massive supply so she expressed a few lots for me. I felt bad asking her to keep expressing for us because she was also a new mum and very tired, even though she was happy to do it.

Not sure if I would use a strangers. I guess if it had come from a milk bank where it had gone through all the quality control tests etc, I would.