Since it's implied from when Gokü was collecting energy for the Genki Dama that Namek has a load of planets next to it, and some of Arale's most extreme gag feats involve planets, Arale has a lot to work with. Whether it is enough to win or not is questionable...
She's likely just come back from Magolor's attacks such as his black hole etc, and probably treat it like nothing, because that's her kind of character, but she has been shown to not exactly be strong against the DBS cast when she partook in the crossover, losing the SSJB Gokü in the beam struggles [although she was "holding back"] and potentially being one shot by Beerus, implying she isn't limitless.
As for who wins? Depends on the win criteria. Magolor can come back over and over before he even reaches his soul form, so complete destruction of opponent will be hard to achieve... However, Magolor has much less to work with on Namek, as he works more with large celestial bodies in space-like environments, giving Arale an advantage if only one KO is required.
So, if an easy to achieve win criteria, Arale
If a hard to acheieve, like defeat completely, then Magolor
If death, Magolor can't have access to his soul form, but since Arale doesn't have much to actually kill him, I guess Magolor would probably kill first.