Hi we're sorry that your colleague can't login to OSM because he forgot password, but you guys already got the only answer available to you.
Account has an e-mail set and it's also linked to google, this means that user can log on Android App via google user and/or on Web version by following the request pass procedure.
We can't disclose the mail address set on the account, but in case user doesn't remember the mail he set on account, we advise him to check all mail addresses he own.
I'm sure he remembers his google account, so he can log via google to check which e-mail he has set.
If he's 100% sure about the mail and he's not getting the reset pass mail, my suggestion is that he can try the forget pass routine by switching between login and mail address.