If your friend is feeding it a lot, it could be waste (poop) that's coming out and not zoox. Is the nem bleaching? If it's been in the tank for a month and it's not bleaching or showing other signs of sickness, then I wouldn't be too concerned.
If it's healthy, maybe you can tell your friend to stop feeding it, and see if it stops expelling the brown liquid.
On a side note, a while ago I posed the question "do anemones actually poop?" and I was chastised for asking, as the assumption is that anything that eats, must remove waste. It appears to be obvious, but my question was if there was any scientific evidence, as in, did anyone analyze what the nem expelled to show that it was actually waste and not zoox or something else? I have 10 nems right now and I haven't seen any of them poop. I assume it just happens in the middle of the night.
If it does only happen at night, and your friend is watching it happen during the day, then maybe it could be indicative of a problem. But, like I said, if it's not bleaching or deflating or showing other signs of illness, I wouldn't worry about it.