Your friend will need to work with Steam for help with recovering their Steam account. Bungie cannot assist with platform account access issues. If access to the account cannot be regained, then it will be permanently locked to their Bungie profile.
Have your friend review the information provided in the article linked below, if they have not already done so.
https://help.bungie.net/hc/en-us/articles/10537372531092
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That is...well, ridiculous to be quite honest. Very poor implementation and functionality of a paid service. But thanks nonetheless I guess.