As “Patur Aval Assur” pointed out, that is the source and the note to that Mekor appears from the Mahari”v, not the Mahari”l as “Daas Yochid” pointed out. Of course, this applies to Ashkenazim as Sephardim have to wait until Monday morning (LaD/34 Omer) to have haircuts.
It appears that the real reason IMHO to permit haircuts on Friday would be that if a person waited until Sunday morning, he would in effect be “slapping Shabbos in the face”, by trimming for a minor holiday and not trimming for (at the very least Chashivus of) Shabbos.
Also, since the influence of any Yom Tov or Shabbos can be ‘felt’ from the Mincha of the day before (as no Tachanun is said), then in theory, a haircut would be ‘permitted’ on Saturday afternoon, but of course that is a Biblical prohibition to say the least, so the two factors seem to point to the heter of Friday haircuts.
It seems to me (to be yotzei all the Deos, aside from waiting until the 49th day as the Ariza”l points out), that a person could have a haircut at the last possible moment on Sunday late afteroon, since he would not be “slapping Shabbos in the face” by waiting until the day (Lag Baomer) would come to an end and at the same time it would still be halachically Lag Baomer. Best of course to check with your local orthodox Rav.