On another lesson using "cabello" I got an answer to your question.
According to this native Spanish speaker, "cabello" is used for the hair on one's scalp and only for humans. "Vello" is hair on the rest of a person's body including one's chin. "Pelo" is used for hair of any kind and on anywhere including animal fur.
"Lavarse" is the infinitive form of the verb. If you wanted to use "lavarse" in this form, you would need to include a properly conjugated form of a verb in front of this. For example:
Ella puede lavarse su cabello. She can wash her hair.
Ella no quiere lavarse su cabello. She doesn't want to wash her hair.
Ella va a lavarse su cabello. She is going to wash her hair.
Hope that helps!