How do you think this could affect Ethiopia?
Assuming the alliance does happen, it will be bound by Portuguese interests in the Red sea, which probably means they want control of the Gulf of Aden, while Ethiopia can have the Eritrean interior.
In the specific context of the Abyssinian-Adalese war, a previous relation could mean an earlier alliance, that could in turn prevent the precarious situation in which Abyssinia found itself when Cristovao da Gama arrived with his musketeers IOTL (Adalese forces raiding the Ethiopian interior). It doesn't need to involve direct military support, though: The early Abyssinian defeats were due to lack of familiarity with firearms, which the Adalese had adquired from the Ottomans and Indian Ocean trade. If active diplomacy and contact with Iberia can be translated into earlier adquisition of musquets and some light cannon, some of the earlier defeats may not happen, or be less severe. Indeed, even with their initial advantage, the war IOTL ended with both states exhausted un terms of resourses and manpower, so an earlier Abyssinian counter-attack in a position of equal equipment would result in a resounding defeat for Adal, short of direct Ottoman intervention.
In this context (Abyssinia neither in shambles nor dire straits) the Portuguese would probably seek an Abyssinian alliance to help them in their ongoing conflict against the Somali sultanates, in exchange for naval help in taking the Adalese coastal enclaves. Note that due to the hilly nature of its territory, Abyssinia was a very feudal state, so the conquest of the lowlands could bring an opportunity for the Emperor to bring more (easily andministrable) land under his personal control, and use them to gain more autonomous influence and start consolidating the general administration (this would require building a personaly loyal bureocracy, though).
Taking into account the Ethiopian mindset of the time though, it is possible that their first impulse might be to reestablish control over western Yemen. But there is another posibility, if the Portuguese manage to provide Abyssinia with enough insentive to allow them to control the Gulf: The main long term failure of Ethiopian empires was that they never managed (or tried really hard, for that matter) to bring a united rule and protection to the regions Christians, implicit to their imperial claim.
A cunning Emperor might see the benefit of the apparently tougher task of consolidating the interior and the Christian Nubian region, while using the Portuguese alliance to secure their flanks in exchange for military support (probably just some manpower) against the Somalis (they most likely won't take any land there, it's the worlds shittiest non-desert land), over the straight forward conquer-more-fertile-but-hilly-and-faraway-land (that will have to be given to very autonomous vassal chiefs, further disempowering the monarchy), while at the same time becoming something that the Portuguese most definetely CAN'T be allied to (a competing Red Sea power).
There is also the obvious: If instead of two near-fatally exhausted medium nations, they find one unified, strong one, the Oromo won't be able to accomplish much.
That's only talking about direct effects, though. Talking about possible effects on the Portuguese, the Ottomans or onyone else would take a whole other post.