I would be inclined to give generic instructions but not provide a direct install. If you install it you will likely be expected to support it including making changes if the host changes its configuration, moving it if the client changes their mind about which host to use, etc. If the hosting arrangement is a VPS or container you may effectively become responsible for securing the VM and actively monitoring it because if a security hole is found unpatched later and used to bring the site down or steal information the blame may come your way for not installing the site securely in the first place.
asking me to login to their dashboard and find what I need
To do this they would need to give you their login credentials. I would refuse from a security PoV: I do not accept other people's credentials. How do you know who else has access to those credentials? How do you know that if they do something untoward (accidentally or deliberately) you won't get the blame as you were the last one known to have logged in? You may even find that him giving his account credentials to you or any other third party is a direct breach of the ToS agreed with the hosting provider. This because less of an issue if the host setup is a reseller arrangement so the client can create a specific account for you, but I doubt that is the case here.
If you don't feel that you can simply refuse...
From a business perspective, perhaps you could point out that installation and support is part of the service you provide with hosting, and that you'd be happy to provide that service on another host for $?? initially and $?? for any support query (not directly relating to your code so already covered by some form of quality guarantee) going forward.
More generally instead of charging for hosting, perhaps tweak your business model and charge for installation, management, maintenance & support and include the actual hosting for free (up to certain bandwidth/space/other limits) or at cost. That way people are free to use another host but it is clear that they have the choice between paying you, asking the host (who will likely refuse or charge), paying someone else, or doing it themselves.