How hard is your playpen to take down? I know our's takes some effort, and good wrist strength, but isn't that hard. I would also recommend taking it down and putting all the baby stuff (toys, large items) back in the nursery/bedroom. Pictures would be left in place. Your friend is in a really sensative place emotionally right now, although I'm sure you are too.
I say this from several angles. I have friends suffering infertility and trying to hang out when my son was tiny was hard on them. I went through infertility myself to finally have him, and pregnant women and small babies were awful for me. Right now, I'm recovering from a miscarriage as well, from about the same timeframe as your friend. Small babies and especially my own son have been big comforts this time, but seeing pregnant women (especially ones that I know have due dates close to when mine was) is painful. I keep quiet about it except to close friends and my husband, unless asked point blank "why aren't you wanting to hang out lately?!" because its my issue, but its still there. Luckily, in my case, the only currently pregnant friend was one of the ones with previous infertility, so she knows without me saying anything.
I think offering to send your child out for the evening was extremely gracious of you. I think taking the large reminders out of the public space is implied with that though, since you do understand why this is hard for them.