I am quite surprised she refused the offer, Even if you are making your own story wouldn't working on something beforehand be valuable experience. In the case of Agrioka manga atleast you have all the characters established and she would only have to continue the story, so she can give more time to Art which is her stronger suit and maybe get in a rhythm how publication of a weekly or a monthly manga works. You get experience working with editors and how the publication works. You even have an initial fanbase which is interested due to the airing anime. There is nothing to lose. Shouldn't Ai-san tell her that if she is working towards a mangaka as a professional, this is a great chance to gain experience, build contacts which may not come again. There are no downsides to the offer. Even if Misusmi san is a high schooler, at least Ai-san should have helped in her decision making. This feels like a stupid decision to me.