Would it be okay if a president in the same situation had unilaterally decided to support Russia instead?
In this case it is unlikely that a US president even matters anymore. Once you “let loose the dogs of war”, they go where they will, and are no longer under your control. A US president is just along for the ride, the war will follow its own logic, or lack of logic.
Europe (its public) is united and committed to supporting the Ukrainians, full stop. If a US president had a different policy he would just have been ignored.