Michaela,
I can't see any reason for
not doing pro bono work for not-for-profits while in between paid jobs.
It is often said that it is easier to get a job when you already have one, so your friend is already at a disadvantage to candidates in paid employment with those employers who stigmatise the unemployed. Engaging in pro bono work cannot make this any worse.
As an employer I would see it as a positive that someone had chosen to do such pro bono work. Not only does it show that they are willing to give something back to the community it also demonstrates initiative, a desire to work and keep their skills current. Many people underestimate how much they can forget about performing day to day tasks when they are not working for an extended period and how damaging this can be to their self confidence. So "keeping your hand in" can be very worthwhile, regardless of whether you are being paid to do it.
The only downside I can see with doing pro bono work is it's potential to impact on the time a person can devote to getting another job. It's also important that it does not impact on availability to attend interviews and start a new job when it is offered.
As for joining AHRI, I have been a member in the past and viewed it as a waste of money. Admittedly I didn't get very 'involved', but I think the vast majority of members don't get involved in AHRI activities, so it is unlikely to be of much help in networking, particularly in the short term.
As for many jobs not being advertised, these are mainly jobs that are being handled by recruitment companies who already have suitable candidates on their database. AHRI isn't going to help with this at all. Your friend would be better off registering with all the recognised
HR recruitment specialists and make themselves available for contract work, which can often lead to a permanent job with the same employer.
Moz.