Well in a sense it can. Personally I think the ascendant is at "risk" of becomeing weak if the individual for instance has the asc 29 degree in let's say scorpio and he/she has his sun and other heavy planets in sagittarius conjunct the asc or in the first. The scorpio asc might then become more sagittarian and even leo-like due to the sun.
I have to ask, does your friend have a planet in the first house? And if so what which planet?

"So my question is can your rising sign become weak depending on the sign the ruling planet is in?"

I haven't looked much into it but I think that could very well be possible. The ascendant is more and less evident within people. Sometimes it seems to have an overwhelming influence and sometimes it's vaguely present.
Although one thing about the ascendant sign that I've concluded is that the sign is expressed a bit differently than having a planet in the sign. Having a cusp in a sign is not exactly the same as having a planet in a sign, even though the sign is evident no matter if it's a cusp or a planet it's expressed a little bit differently.

I should also mention that when the ascendant is located in aries it moves extra fast, so if the birth time isn't very precise and the degree is in the beginning or end of aries your friend might have a pisces or taurus asc.

"Or can a person lean towards acting out the descendant?? "

Very much so. The descendant is a bit downplayed in astrology but it is a definite part of ones "astrological make-up". If the descendant is your apparent characteristics and tendencies, the descendant is the more abstract and hard to pinpoint nature within you that you have a need to project and mirror in someone else and thereby fulfill and see clearly. We kind of seek to fulfill our descendant on a personal level (as oppose to the midheaven that has more to do with our self image and official persona that doesn't have as much to do with the personal self).

if you're asking if the sign on the AC is always noticeable or not, I'd say yes, it's possible for the Ascendant to be "weak", but in reality it never falls in the background of one's "perception", since it is the main "filter" between us-as bundles of stellar energy- and the "real world"...

But, obviously, it can be overshadowed by stronger influences in the chart. Personally, I always saw my Virgo Rising as my "conscience"-like the little angle hovering over my shoulder, whispering nasty things in my ear every time I stray from "the righteous path"-which, in my case, is paved by the rules of Virgo, of course.

And I would say it has a surprising amount of influence, despite the fact that the rest of my chart is formed of energies which square it(mostly Pisces and Gemini)...I truly admire Virgo traits, above all other traits, and always try desperately-at least part of me does-to fit the Virgo mold of being a "Machine".

I would say it can become kind of 'weak' in the personality, because I'm also an Aries rising but I don't really show it much either (unless I'm mad, but it's pretty rare for me to get too riled up though lol). I think because I have so much Pisces influence (Sun, Mercury, and Venus), it kind of overrules my Aries tendencies lol.

"Can a person's ascendant be weak in their personality? For instance my bestfriend is a rising Aries, I would not have thought she was because she is not agressive, she does not have a me first attitude etc...BUT her mars is in libra-aries opposite which makes her passive aggressive and she always gives into others first and never herself. "

I'm not entirely convinced that this is an example of someone with a weak rising sign. Certainly one of the traits of an Aries Rising is to approach the world with an aggressive streak, however that is only ONE manifestation of Aries Rising. Another might be a sense of chivalry, a sense of defending the underdog, of being a self-starter, independant and an catalyst for change. You can do all these things and not have a 'me first' attitude or even be aggressive. with Mars in Libra perhaps this energy is just more balanced rather than weaker.

I think the ascendant might play the immediate and daily role in our conscience while saturn for instance is the long-term, stretched out part of the conscience. The part of our conscience that deals with the more general aspects of how we lead our lives with a larger time span taken in account.

So the asc is our daily guilt and saturn is our guilt of the decade ;)