Risks of subletting?

[Jerry Rubinow]

I'm renting a house (in Mass. if it matters) to a group of three tenant, three friends on the same lease.  One of them just informed me she is purchasing a house for herself and wants to know if she can sublet.  The existing lease requires the written approval of the landlord if there's going to be a subletter.  What are the risks and other things to consider with this?  I'm a new landlord and don't have experience with subletting.  Questions that come to mind:

1. is there any paperwork or legal documents for me for a subletter, or is all the burden on the existing tenant?
2. is the original tenant legally responsible for rent payment to me and not the subletter?
3. if there is a problem with the subletter with rent payment or they don't want to leave at the end of the lease, are there any additional legal complications with eviction than with a regular tenant?
4. Anything else?

The tenant who wants to sublease would do the work of finding the subletter, and I'd require the subletter to go through the same screening the original tenants did, of course.

I suppose the other option is to swap out the potential subletter as a full tenant on the lease.  

Any info or advice appreciated - Thanks.

-Jerry