Granted we only have 17 but still, I’ve never used anything except a simple excel spreadsheet and all my rental properties have a separate savings account under my main account at my local bank.

But if they gov't changes to a 90 day minimum, you would have to operate as a 90 day rental or longer.And talking to local experts, if you want to own a STR on O'ahu, your safest bet is to buy a unit in a Condotel that is located in the "Resort" zone of Waikiki.  

One tip: focus on areas with year-round demand and check local regulations early—they can make or break a deal. 

Changes to local landlord/tenant laws and new situations involving tenants lead to these annual updates.

Attend the local and regional Real Estate meetups.

Try some local small banks or look in the tools on BP

I would outline your scenario to a few local mortgage brokers - with everything you've said this seems like a pretty easy deal. 

In Pittsburgh, where I live and operate, this would be a two to three-month process in front of almost all out local district magistrates.

Ensure the sublease is explicitly permitted in a written agreement with your friend and aligns with local STR laws.

The front end can sometimes be tedious to get approved by the local housing authority, but rent payments are always timely.