Note, that HMRE was publshed before RoP:F and both books have authors in common. So referring to HMRE to understand RoP:F is necessary.
Well, we have in it:
Gifted characters can learn Supernatural Abilities as described on p.166 of Ars Magica Fifth Edition. Their favored Abilities are: Curse-Throwing, Dowsing, Summoning and Second Sight.
So the 'Their' refers to Gifted faerie doctors. Also because for the unGifted ones the distinction of 'Favored' - as described in HMRE p.9f Favored Abilities - makes no sense.
In HMRE p.9f we read:
Some traditions have a set of favored Supernatural Abilities. Gifted characters who belong to these traditions do not have to subtract the total of their Supernatural Ability scores from the source quality to learn other Abilities that are favored by their tradition.
Usually, the character has to undergo some sort of ritual - Opening the Gift - before he gains this benefit of their tradtion. Favored Abilities have no meaning for unGifted characters, as they cannot learn Supernatural Abilities ar all.
Compare this to RoP:F p.117, and you find:
(1) Faerie doctors are a tradition with Favored Abilities for their Gifted members.
(2) They may or may not have rituals for Opening the Gift: we are just not told.
I have trouble imagining an unGifted faerie doctor uncle performing Opening the Gift for a season on his Gifted nephew. As such a ritual is not necessary, I'd better do without it.
Still, I would consider an informal Opening the Gift working on Gifted faerie doctors for the purpose of HMRE p.8 box Opening the Arts and the learning of other Supernatural Abilities not favored (see also p.8). This may well be the effect of having been trained from birth by a relative as a faerie doctor (RoP:F p.117).