I'd tell my friend a bit about what they can expect, how aro people in general experience the world, what it feels like, about romance repulsion, QPRs, etc.
Every aro person is different, so my aro experience isn't neccessarily the same as this other person's. What I'd like is quite different from what other aros would choose, so its important to prepare them for different scenarios.
Its most likely that this aro person will not be open to dating them, but perhaps would be into something else.
Does 'interested' mean your friend has a crush on them, or in love with them? Would they be interested only in dating this other aro, or are they more open minded?
If they don't get exactly what they expected, would they loose interest?
The other person's sexuality definitely matters, because its a factor in relationships. I'm not sure how much you told your friend already, but a bit of info would not hurt. I'd ask their opinon on poly things, just in case.
Also, I'd say this other aro person is quite fortunate to have someone else around to educate your friend.
Most allo people who pursue aros have just no idea, and especially if they aren't getting the conventional 'yes' or 'no' messages, they get angry and confused. To many of them a 'yes, but not that' sounds like you are playing with them, lying, or wasting their time. You can save them a lot of awkwardness, misunderstandings and dissappointmet.
But no, I would not tell them to give up. Just to keep an open mind.