。
She has students living as lodgers.
Literal
She [topic-は] students [object-を] is-causing-to-lodge.
下宿させる is the causative of 下宿する ('to lodge / board'), literally 'cause [students] to lodge' — i.e. 'take in lodgers.' Layered with ~ている, the construction expresses the ongoing arrangement: she is currently in the state of housing students. The causative + ている combination is a tidy way to say 'be hosting / be having someone do X' as a continuing situation. Note that させる here doesn't carry coercion: when applied to actions where the causee benefits or willingly participates, the causative shades toward 'allow / let' rather than 'force,' since lodging students is a transaction both sides choose.