It is not a couple of 13 year olds. One is older. Blow jobs are not low risk. They spread HPV, which causes cervical cancer in women and cancers of the mouth and throat in both women and men. Cancers of the mouth and throat are on the increase partly thanks to ignorance of the dangers.
A condom will not guard against HPV. Unless a young person is vaccinated, all sex carries with it the risk of contracting or spreading HPV.
Does DS know for certain that his friend has not been having sex with others besides himself?
I am aghast at your idea that 'then she may feel the need to tell the whole school' is a serious reason not to give the girlfriend information she will need in order to make a necessary decision about her health. The girl's parents need to be told that the DS has possibly exposed her to HPV (yes, even through kissing) and that she needs to be tested.
They can then think about the vaccination.
Most certainly, the DS needs to be vaccinated, especially if he intends to continue having sexual encounters with boys or men, but also because he has already engaged in sex with another boy and because he may also or instead intend to have sex with girls/women.
Girls and women deserve all the information that is available in order to make sound healthcare decisions.
The DS should be encouraged by the OP to tell the girlfriend the truth about his relationship with his 'friend'.
To let him two-time her this way is awful.
He is not entitled in any way to have his cake and eat it where this girl is concerned.