Essentially you can't

It sounds like this senior developer is essentially taking the opportunity to not do their work. As an intern there really isn't anything you can do about that directly. Really the senior dev is only punishing themselves by doing this since they will have to pick up an unfamilar codebase very quickly after you've gone, although they might try to blame any problems on you interns (since you won't be around to defend yourselves) but I would expect a good manager to see through that.

You could make his manager aware that he isn't following the manager's instructions but that will be difficult to do since the manager is on holiday (if I understand you correctly) and this isn't something you probably want to do purely over e-mail. If you have any overlap with the manager before leaving you could discuss it with him but depending on the manager's current perception of the senior dev this could backfire on you so I would approach such things with caution - you wouldn't want to say "[Senior Dev]" slacked off while you were gone" is probably a bad thing to say "[Senior Dev] didn't get much chance you work with us while you were gone" is much better since this lets the manager draw their own conclusions as to whether the senior dev was slacking or not.

Essentially you can't

It sounds like this senior developer is essentially taking the opportunity to not do their work. As an intern there really isn't anything you can do about that directly. Really the senior dev is only punishing themselves by doing this since they will have to pick up an unfamiliar codebase very quickly after you've gone, although they might try to blame any problems on you interns (since you won't be around to defend yourselves) but I would expect a good manager to see through that.

You could make his manager aware that he isn't following the manager's instructions but that will be difficult to do since the manager is on holiday (if I understand you correctly) and this isn't something you probably want to do purely over e-mail. If you have any overlap with the manager before leaving you could discuss it with him but depending on the manager's current perception of the senior dev this could backfire on you so I would approach such things with caution. "[Senior Dev]" slacked off while you were gone" is probably a bad thing to say. "[Senior Dev] didn't get much chance to work with us while you were gone" is much better since this lets the manager draw their own conclusions as to whether the senior dev was slacking or not.

Essentially you can't

It sounds like this senior developer is essentially taking the opportunity to not do their work. As an intern there really isn't anything you can do about that directly. In theory the senior dev is only punishing themselves since they will have to pick up an unfamilar codebase very quickly after you've gone although they might try to blame any problems on you interns (since you won't be around to defend yourselves) but I would expect a good manager to see through that.

You could make his manager aware that he isn't following the manager's instructions but that will be difficult to do since the manager is on holiday (if I understand you correctly) and this isn't something you probably want to do purely over e-mail. If you have any overlap with the manager before leaving you could discuss it with him but depending on the manager's current perception of the senior dev this could backfire on you so I would approach such things with caution - you wouldn't want to say "[Senior Dev]" slacked off while you were gone" is probably a bad thing to say "[Senior Dev] didn't get much chance you work with us while you were gone" is much better since this lets the manager draw their own conclusions as to whether the senior dev was slacking or not.

Essentially you can't

It sounds like this senior developer is essentially taking the opportunity to not do their work. As an intern there really isn't anything you can do about that directly. Really the senior dev is only punishing themselves by doing this since they will have to pick up an unfamilar codebase very quickly after you've gone, although they might try to blame any problems on you interns (since you won't be around to defend yourselves) but I would expect a good manager to see through that.

You could make his manager aware that he isn't following the manager's instructions but that will be difficult to do since the manager is on holiday (if I understand you correctly) and this isn't something you probably want to do purely over e-mail. If you have any overlap with the manager before leaving you could discuss it with him but depending on the manager's current perception of the senior dev this could backfire on you so I would approach such things with caution - you wouldn't want to say "[Senior Dev]" slacked off while you were gone" is probably a bad thing to say "[Senior Dev] didn't get much chance you work with us while you were gone" is much better since this lets the manager draw their own conclusions as to whether the senior dev was slacking or not.