DRG ARR, UNITED FLT 282 (BOEING 727, N7255U) WAS VECTORED BY LOS ANGELES (LAX) TRACON FOR AN ILS RWY 25L APCH. WHILE IN A DSCNDG TURN AT 2100' MSL TO INTERCEPT THE ILS LOC AT 2000', THE CAPT SAW A HELICOPTER (HEL) AHEAD & PULLED UP. HE ESTDHIS ACFT PASSED W/I 200' TO 300' OF THE HEL. THE HEL WAS A DEA ACFT (HUGHES 369D, N8620F) ON A SURVEILLANCE FLT. THE DEAPLT SAID HE WAS HOVERING BLO THE LAX TERMINAL CTL AREA (TCA) WHEN HE SAW THE BOEING MAKE A RGT TURN TWD THE LAX ARPT. HE SAID THE BOEING WAS IN HIS VIEW FOR THE ENTIRE TIME & IT WAS NOT A HAZARD TO HIM, BUT HE WAS CONCERNED ABOUT ITS LOW ALT. LATER, HE ADZD LAX TWR. THE DEA PLT STATED HE WAS OPERG BTN 1400' & 1900' & THAT THE HEL WAS AT 1300' WHEN THE BOEING CONVERGED. HE ESTD THE BOEING CROSSED 300' TO 400' ABV THE HEL. RADAR DATA INDCD THE 2 ACFT CONVERGED TO A SLANT RANGE OF 1500'. MIN VERT SEPN WAS 200'. RADAR DATA ALSO INDCD THAT FLT 282 REMAINED ABV THE TCA FLOOR (2000') & THAT THE HEL WAS BLO THE TCA. A LEAK WAS FND IN THE HEL'S PITOT STATIC SYS, WHICH CAUSED AN UNDETERMINED ERROR.