If he thinks that filling some grease into the boots is doing anything - dont go to this mechanic, The grease needs to be in the ball cavity of the ball joint and greaseable joints have a zerk (grease nipple) on them where you attach a grease gun. Such ball joint boots were not sealed either - you had to pump the grease into them till you see fresh grease ooze out of the boots, This practice was done every 6 months or so, newer type semi sealed ball joint boots and joint having provision of greaseing - they take a pump or 2 every year
Modern cars no longer have grease zerks on them - the component is replaced when worn out.
ideally a moly additive chassis grease is used in ball joints, something with NLGI-2 rating and should carry a GC-LB rating. Such greases are extreme pressure greases.