Cambridge has great public transportation-- a car will be ticketed or towed within the first week (the parking regulations in Cambridge are more complicated than they look…)
She should reach out to HR at the company she’s working for to get the names and contact information of the other interns or other new hires. She may be able to “tag along” on someone else’s sublet!
She can live in Allston, Boston, Brookline, Somerville, Brighton, West Roxbury just to give you some names of close in neighborhoods that she can start to research. All of them have a lot of student housing which is very transient-- some leases run September to September (so she’d the get the tail end of a student moving out) and others run June to June (so if there’s someone starting at her company in September but needs to sign and pay for the lease now, she’d get the early months).
I wouldn’t worry about A/C. Commencement month in Boston (runs from mid-May to first week of June, Harvard is usually last) you see A/C units sitting on the street on trash day. Consider it a form of recycling!
What is her move in day? May is rarely hot enough to need AC and sometimes it won’t get hot until end of June…so she can pick up a unit once she’s there.