Using Intervention on a high-DPS ally can turn the tides of battle by giving your ally free reign to attack.
Kayle can effectively do large amounts of damage or support her allies depending on her item build.
Kayle benefits greatly from Attack Damage and Ability Power, making hybrid items like Nashor's Tooth and Trinity Force very effective on her.
If you wish to support your allies, items like Locket of the Iron Solari can help you assist teammates.
Intervention only grants Kayle or her allies immunity to damage. Slows and stuns can still be applied, so trap them if they're being overzealous.
Kayle is usually a fragile target. If you see her ultimate cast on an ally, switch to attacking her.
Be sure to look at what items Kayle is building to determine what role she is fulfilling on her team.