If the boot can take it why not? YMMV. In my experience with the S40 it's very common for the inner boot to go before the outer. For this reason alone I'd pull the axle & do both. It's also easier to see & clean thoroughly.
Make sure it's the axle by cleaning the inner or outer boot and looking for a tear. It can also (though not as likely) be the passenger side engine mount leaking fluid down the chassis - then the axle flings the oil everywhere.