What you are looking at is likely Herpes Simplex Virus Type 1 (HSV-1). Commonly known as Oral Herpes. Just like he said, he probably got it from that one kiss.
And the only mistake this brother made was judging based on outward looks.
You see ehn, imagine your mouth is a house.
Oral herpes is like a stubborn visitor that enters the house when you kiss someone who has it (even if they don't have a sore on their mouth at that time!).
Once the virus enters, it travels to the nerves (the wiring of the house) and goes to sleep.
Sometimes, when you are stressed, sick, or under the sun, the visitor wakes up and causes riot on the lips. That is the blister/sore you see.
Once this visitor enters your house, he NEVER leaves. There is no cure. He lives there rent-free forever. You can only manage him so he doesn't cause riot often.
Is it dangerous?
Usually, no. It’s annoying and painful, but it won't kill you.
But you must be careful not to kiss babies or touch your eyes after touching the sore, because that one can cause serious problem.
For this brother:
1. He needs to go to the hospital and get m antiviral medication (like Acyclovir) to help the healing.
2. Do NOT pop those blisters. The water inside is pure virus.
3. Take it easy.
Lesson for everyone: You cannot know who has Herpes by looking at their fine face.
Many people have it and don't know. Please always ask questions before you lock lips.