Most real estate coaches will advise you to prequalify and pre-approve before you ever put buyers in your car. A 5 star client is open, honest and wants your help. That includes answering all your own pre-qualifying questions as well as the lenders.
Once you explain how it saves time, stress and probably money to be pre-approved before they go looking, it should begin to make sense to them - unless there are issues they have not shared with you.
Folks who are offended by your request to help them get pre-approved are probably not going to be the best use of your time. Worse, they take away time that you could use to help other people who would benefit from your skills. Ask your own questions and choose carefully the clients you spend your time and energy with.