It is hard to believe your buyer cannot find where/how to give you a tip.
Just two days ago, I placed an order with my favorite artist on Fiverr to alter my profile picture. She got the work done within hours.
When I went to complete the order, I was told within the process that it was “customary to tip a buyer.” This message was followed by four choices from which I could choose. They were $5, $10, custom tip, tip later. I chose “tip later.” Then I was brought to the rating page. After that, I was AGAIN reminded to leave a tip, and the same four choices were presented to me. Again, I skipped and ignored the reminder to leave a tip.
The next day I got an email from Fiverr asking me to submit a secret review of the seller. I did that, and once again, the message popped up about leaving a tip. Once more, I chose not to do so.
Later that day, I got a message through my inbox. For the 3rd time, I was asked my opinion about the seller and to tick the boxes of my choice. Then for the FOURTH time, I was reminded it was customary to leave a tip. UGH!
So, since Fiverr is working hard to get a tip for us, I do not see how your buyer can say he could not find the tip option because I found it took a great deal of time NOT to leave a tip for my seller.
Please Note: Typically, I do tip sellers, and this seller deserved a tip, but I was curious to find out how far Fiverr would take the tipping thing as I had heard buyers were asked two times to leave tips. It ended up that all of the reminders put me off about leaving a tip.