With depression and anxiety you already have a full plate and I would definitely advise against volunteering to become your friend's representative payee. Can you guarantee that your own health will never impact your duties? Can you afford the financial exposure if it's decided you used funds improperly and are liable to pay them back out of your own pocket, even though you can't collect a dime in exchange for all the work you'll put in? If he or she don't have a family member that would be trustworthy, a professional service might be a more appropriate option.

It's been reported that a person improving enough to no longer need a representative payee has been used to show evidence that they have improved enough to have their benefits terminated. Should you decide to go forward, "SSA requires you to complete the payee application in a face-to-face interview (with certain exceptions)." Whether an SS employee would question if you've improved and you might get stuck undergoing a long form CDR is at least a possibility. Or what if you open up a different can of worms and the SS employee decides you need a representative payee yourself? Add in that you could end up ruining your friendship if the person becomes resentful of your decisions, this just seems like there is substantial risk for you on many fronts.

Here is the list of what your required duties would be:
"A payee acts on behalf of the beneficiary. A payee is responsible for everything related to benefits that a capable beneficiary would do for himself or herself. SSA encourages payees to go beyond just managing finances and to be actively involved in the beneficiary’s life. The following lists the required duties of a payee.

Required Duties:

Determine the beneficiary’s needs and use his or her payments to meet those needs;
Save any money left after meeting the beneficiary’s current needs in an interest bearing account or savings bonds for the beneficiary's future needs;
Report any changes or events which could affect the beneficiary’s eligibility for benefits or payment amount;
Keep records of all payments received and how they are spent and/or saved;
Provide benefit information to social service agencies or medical facilities that serve the beneficiary;
Help the beneficiary get medical treatment when necessary;
Notify SSA of any changes in your (the payee's) circumstances that would affect your performance or continuing as payee;
Complete written reports accounting for the use of funds; and
Return any payments to which the beneficiary is not entitled to SSA."

There are some webinars at this link that might be helpful: http://www.ssa.gov/payee/index.htm#sb=2