pnp - excessive handwashing can definitely be a part of OCD, and OCD can definitely be comorbid with ASD.
The difference needs to be teased out by a psych- OCD results from obsessive thoughts leading to compulsive behaviours. My 17 yr old with Aspergers was diagnosed with comorbid OCD when he was 11. He also handwashes, only with cold water and liquid soap, though he will use a cake of soap if nothing else is available.
Most rituals/stimming behaviours associated with ASD are less complex than handwashing, but it can be a stim. I am not sure that handwashing could be classed as an obsessive interest, but again, anything is possible
If she is handwashing because she is worried her hands are dirty, you need to have OCD considered, otherwise try distracting her. If she starts to conceal what she is doing - then she needs to be assessed for OCD.
cheers
Rae