Many service provides have clients sign up to a maintenance plan which includes the licences used.
It depends how you've explained everything to your customer, if they are fully aware of Elementor and how it works then explain its a premium script and if they don't have a maintenance plan for the site then they will need to purchase a licence.
That said, We've got maybe 20 customers on our Agency plan which dont have maintenance plans, they are tiny businesses on 1 - 3 page websites and to be completely honest, they will continue to use our licence as long as they wish, it doesnt cost us anything and it providers a good service.
In some cases we even have some businesses added to our maintenance plans free of charge because the sites are built in such way that its rare they would ever have a problem, if they are hosted on our servers, we'd rather they were maintained and updated all the time as long term it saves alot of bother and again its great customer service.
We mainly allow customers to use our licences if they host on our servers, have maintenance plans, have SEO plans or are long term customers. It's just good customer services and with licences holding 1000 installs we will more than make enough money from those 1000 installs to compensate for the few "good will" clients we allow to use it.
Providing you are in the business for the long haul and plan on renewing your licences, then i dont see a problem being good to your customers. If however you dont think you'll be around and wont renew your licence, then you should make your customers aware and make them buy their own.
