Snorky, consult a real estate attorney for free (not a tenant's attorney or anyone affiliated with the Tenant's Union), present them with all the information and get an estimate on total cost and length to do an Ellis Act eviction on this unit or the entire building.
With the lawyer costs in mind, suggest to your parents that they go directly to these tenants and negotiate a higher rent going forward. If these people are friends, then negotiation and coercion through tenant guilt may produce better fruit than bombing them with legal papers right out of the gate.
If the tenants refuse to pay more rent, see if they will negotiate a payment for departure. You could offer them 1/2 of the lawyer's cost if they were to leave within 3 months. I would consider even offering them all of their rent back ($8-10K). Landlords are paying tenants twice that in order to help their buildings sell with empty units.
If the tenants don't want to negotiate, increase their rent to the banked amount immediately, threaten to Ellis Act (even if the unit/building is ineligible), and then threaten to sell the building to someone who
will Ellis Act these tenants and flip the property.
If your parents are elderly and gullible, it might be best for them to make you the 'property manager' and you can act as their agent without title transfer (which carries negative tax results). You can take over where their negotiations end and start dealing with the tenants yourselves.
P.S. I am not an attorney but you should speak with one before you or your parents do anything.