The Ascendant ruler is not necessarily the most important planet in a chart - there's a whole history of attempting to assess the 'Almuten of the chart', 'Lord of the Geniture' or 'Chart Ruler'. Planets' in the first, especially one so close to the Ascendant have to be considered (as do other angular planets). So your friend may well have two (or more) dominant planets. I have three angular planets, (Saturn conjunct the Ascendant, Moon conjunct the Descendant, and Venus conjunct the IC) not to mention Mercury in the fourth. These three seem to be more influential than my third house Sun, which rules the Ascendant, on a day to day basis. However, the Sun does set the tone of my life
I can only speak for myself here. Firstly placement by house is more important than placement by sign, when it comes to reading a chart. That doesn't mean that signs are unimportant, or can be ignored entirely. I use the sign placement to judge how strong a planet is 'in itself' that is the ease or otherwise with which it will influence the area of life associated with its house placement. Leave the house positions and indeed the degree positions of my planets as the are, but revolve it half a revolution and I'd have two planets in rulership and another in exaltation - life would be much easier, and I'd have a better chance of realising my potential.
So yes the Ascendant ruler is defined by what sign the Ascendant falls in - by definition - and it is important, for good or ill. You can also, if you wish, look to see if one planet stands out in a chart (not necessarily the Ascendant ruler) and call that planet the 'chart ruler'. There's no agreed way of doing that, 2000 years of history has seen quite a few variations on the calculation but it can be useful. In fact I think we once had a thread on it sometime back.