A friend of mine, who did some investing back then made a deal with a co-worker to add his name to a bank account with his co-worker so if any one asks, he had seasoned cash for a number of years to show.

Its definitely a judgement call but depending on exactly what bad credit is  (vs. say no credit)  a co-signer or guarantor may be an option.   

Example, TIC established with Syndication entity LLC owns 90% of property and investor would own 10% using value from a 1031-exchange in purchase.In this case, the 1031 exchange investor becomes a "co-owner" of the property and not really a "limited partner" in the syndication entity.

If the tenant does not have credit to rent then needs a co-signer and they need to be verified and approved and I would explain that to the tenant and get the phone numbers of the co-signer and explain it to them as well. 

Just "divorced" about 8 months ago, with a co-owned house (hoping to get my name off the house soon).

Getting a co-signer?

We own a co-op but will likely move to a nearby NY county (Westchester/Rockland) in a few years' time.

It should always be done when you are under contract on a property to make sure there are no issues that will pop up prior to closing with a CO.

I can use my sister as a co-signer.

If you're still uncertain, you can ask for a co-signer.