AD&D is mostly a system for determining what happens in situations we already understand, rather than dictating what situations are possible to experience in the game. There is no reason why a stealthy 3rd party could not approach a fight undetected, therefore it is possible.
You can determine whether there is surprise by making the usual roll: 2 in 6 for each of the three groups (the PCs, gnolls, and lizardman sneak), modified by the situation and by the abilities of the groups.
Unfortunately, AD&D doesn't provide any effects of such a situation.
Normally, surprise is modelled by making one side inactive for a number of segments, and giving the unsurprised side particular advantages during that time (DMG, p. 61–2). This obviously doesn't work when the surprised party or parties are actively fighting each other — it wouldn't make sense to model surprise as making them pause while the new, unnoticed party acted.
In practice, as is often the case in AD&D where it doesn't provide a method of handling something, it means that the DM has to make a judgement about how to handle it. It can be as simple as just declaring an event mid-combat: “Fighter, you ward off the blows of the largest gnoll, only to be suddenly attacked from your flank…!” (at which point the DM makes the necessary rolls and narrates the outcome of the attack). The timing would be based on the DM's knowledge of the lizardman's movements, the new surprise rolls (made in secret), and the factors in Distance on page 62. It's entirely possible for a successful surprise by the lizardman to gain no advantage due to the particulars of the situation, or to gain as much as one or more full set of attacks on its victim.
The DM has a responsibility to be accurate but fair in this, but that's par for the course in AD&D.