I think this is not wpForo issue.

1. Please make sure your server email sender works well. Make sure you got emails from your WordPress (on registration or on other cases...).
2. Receiver is the same sender. Make sure the "From Email" (Dashboard > Forums > Settings > Email) is not the same subscriber or new user email.
3. If the "From Email" domain doesn't have website domain name (e.g.: info@gmail.com, but website is example.com) it may tracked as spam, you should check in spam box.
4. Some hosting services, or some Antispam plugins try to cache emails and those are not being sent immediately. You should wait 1-30 min (e.g.: on some GoDaddy hosting plans you receive emails in 45 min).
5. If you're using GMail Fetch services, go to Gmail Settings > Accounts and Import and click on "Check mail now" link to load all new comments from website email server.
6. Test it as real as possible. You should use new IP, new emails and new accounts. The best way to ask your friend to post a reply with Subscription and Subscription Confirmation. Then reply him.