Adding Temporary Guest to Lease? (Massachusetts)

[Christopher Reynolds]

Hello everyone.  We have tenants who moved from another part of the country to live in our 3 family building.  They mentioned having a daughter that was moving up here too, and would be staying for a few weeks when she arrived here.  I wasn't present for the lease signing (my ex-GF and co-owner/landlord) was present, and they said it was for a couple months, and "we told him all about it".  I wasn't happy they exaggerated about it, but whatever.  We had them write a note saying she would stay with them until February 28th, and we took her ID and other information.

Today, we receive a text message from one of the tenants saying that "we need to add daughter to lease as she's unable to find permanent housing, let us know how to do this as soon as possible".  We are not interested in having her move in permanently, especially at the current rate.  Only the two tenants are on the lease, their daughter is not mentioned in it.

What to do?  My ex GF still lives in one of the units, and might not feel comfortable to tell the tenants what I would like to tell them (please go find some sand and pound it), but we surely don't want them to assume they can move in another adult into the unit without any changes.  Has anyone encountered a situation like this?  Thanks much!