First, the Slaying Longbow is listed under the Actions section, and there is no equipment listed for the Solar, so it is up to the DM whether the Solar uses persistent weapons, or forms them at the time of attack, meaning there may not be any weapon to loot.
As for the Slaying Longbow attack, there are 3 parts, attack, damage, and special:
Ranged Weapon Attack: +13 to hit, range 150/600 ft., one target.
As in any other attacks in 5e, the + to hit is a result of the creature itself and any listed bonuses, none of which are present, so you would only use the range and target traits.
Hit: 15 (2d8 + 6) piercing damage plus 27 (6d8) radiant damage.
As in any other attack, this lists the average, die, and ability modifier damage done by this weapon (dependent upon the creature), but also lists bonus radiant damage, which is the result of one of the Solar's traits:
Angelic Weapons. The solar's weapon attacks are magical. When the
solar hits with any weapon, the weapon deals an extra 6d8 radiant
damage (included in the attack).
This indicates that the weapon itself does not do the additional 6d8 radiant damage, so you would not be able to make use of it.
In addition, there is a special section of the attack not present elsewhere:
If the target is a creature that has 100 hit points or fewer, it must
succeed on a DC 15 Constitution saving throw or die.
Which is likely a special attack trait of the Solar, that is not present within the weapon.
Now, assuming the attack is made with a persistent longbow that can be looted, the only info we have for it is a damage of 2d8 piercing, meaning you would not have access to any special properties.
In addition, because the Solar is of size Large, the longbow might be oversized.
Applying all of the above, if the longbow is capable of being looted, you would have a ranged weapon that does 2d8 + Dexterity modifier damage, and which may be oversized.
Finally, it would of course be up to your DM if you can loot the weapon, what properties it has, and if it is magical or not.