How can I prevent unauthorized access to home Wi-Fi using iPhone fingerprint unlock?

That has nothing to do with your fingerprint. You entered your Wifi Password on his phone, so now it is saved there so he can reconnect. You have 2 options:

• Go on his device and select your Wifi, then choose "Forget this Network"
• Change the WiFi password on your router.

From your post it said "I did unlocked his phone to put the wifi password", which sounded like you put the WiFi Password in. At any case change the WiFi Password on your router and the problem is solved. There are many ways to get the WiFi password and I cannot begin to guess how that happened, but I can guarantee it was not from your fingerprint that you put on his phone. Just a couple of examples for getting the WiFi password are:

• With some Internet/TV providers, you can simply use the TV Remote and ask "What is my WiFi Password" and it will display on your TV.
• Most routers have a default WiFi name and password stamped right on the router.
• If he ever had access to your phone when it was open or knows your Passcode, the WiFi password can easily be shared to another device. If he allowed you to have your fingerprint on his phone, it would not seem plausible that he would have never had your phone in his hands when it was unlocked, even if he did not know your passcode to unlock it himself.

How to share your Wi-Fi password from your iPhone, iPad, or iPod touch - Apple Support

"How can I prevent unauthorized access to home Wi-Fi using iPhone fingerprint unlock?: Only relevant thing here could be, I had my finger print access on his phone and I did unlocked his phone to put the wifi password. there is no friend or family group created on my iphone"

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Securing your Network:

Just how many devices do you have on your account? If only a few, then changing the network's password may be the best route to take. Log into it by typing the Default Gateway in your Web browser's address bar and then pressing return or enter on the keyboard. Reply back, if you need help with that.

For future reference...

Create a Guest Network with high restrictions, when you need a friend access to your network connection. Did so for a neighbor. Look up how to do so in the User Manual of the router.