My girl has a bad knee as well. There’s some discussion in this thread:
When I was going through PT for a knee injury (quad tendon, not patella), they had me do EKGs (if I remember correctly). You just stand against a wall w/ a soft ball behind the knee, straighten the leg against the ball, hold, and repeat. Along w/ that (in order of introduction): elliptical, wall slides, foam rolling, hip thrusts, single leg press/extension/curl, some hip work w/ a band, and finally forward lunges.
Using a bike might hurt your knees because it’s not properly adjusted (knees going out too far in front).