Not 3/91. 91/3....account opened at least 91 days, and 3 statements have generated on said account. If the two factors exist, you should be in a good place to make the request. However, there are other factors. Utilization, income, DTI, etc.
The only way an AU can hinder your credit is if you, the primary, isn't controlling their spend. Or, depending on your arrangements, they don't pay; consequently making you responsible. There are, of course, other factors with this as well. Overall UTIL and other TLs reporting to name a few.
Personally, I'd first consider if this person is trustworthy. If all else fails, NFCU allows the primary to enforce spending limits on an AU. Your SL is only $1k. If you're worried about it, enforce a $200 - $300 spending limit. You're controlling your own UTIL destiny as well as your risk of exposure.
Good luck!