[paxys]

You don't need power of attorney to act on someone's behalf with their explicit permission and instruction. It may be against TreasuryDirect's terms of service (I have no idea, I haven't read them), but there's nothing specifically illegal about this.

[dpifke]

Interestingly, TreasuryDirect's terms of service are codified in the Code of Federal Regulations:

https://www.law.cornell.edu/cfr/text/31/part-363/subpart-B

From a brief reading, having a third party access your TreasuryDirect account does seem problematic:

https://www.law.cornell.edu/cfr/text/31/363.22

https://www.law.cornell.edu/cfr/text/31/363.17