Another probably simple question I just remembered :)
Is there some person in D/P that is similar to the lady in Slateport City in R/S/E that gives your pokemon a ribbon for having used all of its 510 EVs?
Yes, she's in the last city (Forgot the name, I'm tired) in the building with the Seal vendor.
Also, if you want to EV train against trainers, here's a list I found compiled on GameFAQs:
HP:
Route 209 - Cowgirl (?) - L14, 15, 15, 15, and 16 Bippa (5 HP)
Route 210 - Bird Tamer (lower level) - L38 and 42 Noctowl (4 HP)
Route 212 - Fisherman (lower one) - L17, 17, and 18 Barboach, L20 Karanakushi (4 HP)
Route 213 - Male Tuber (pacing) - L52, 53, and 51 Toritoidon (6 HP)
Route 219 – Female Tuber – L29 and 29 Marill (4 HP)*
*Not recommended because she’s on a very small island with confined movement.
Attack:
Route 203 - Lass (upper area) - L38 Mukkuhawk, L38 Biidaru (5 Attack)
Route 214 - Gambler (? guy flipping coin) - L36 Goldeen, L37 and 38 Seaking (5 Attack)
Route 218 - Fisherman (bottom one) - L53 and 53 Gyarados (4 Attack)
Route 222 – Fisherman (leftmost) – L53 and 53 Gyarados (4 Attack)
Route 225 – Cooltrainer (standing still) – L53 Pinsir and L53 Mightyena (4 Attack)
Defense:
Route 210 (tall grass section) - Ninja Boy (middle) - L23, 23, and 26 Geodude (3 Defense)
Route 212 - Collector - L17, 18, and 19 Geodude (3 Defense)
Route 221 – Collector – L51, 52, and 53 Hippopotasu (3 Defense)
Special Attack:
Route 212 (rainy section) - Male Pokemon Ranger (northwest end) - L21 and 21 Pottaishi (4 Sp. Attack)
Route 212 (rainy section) - Scientist (?) - L53 and 53 Alakazam (6 Sp. Attack)
Route 214 - Male Psychic (spinning, pacing) - L38 Haunter, L42 Kadabra (4 Sp. Attack)
Route 214 - Collector (right path) - L20, 22, and 24 Roselia (6 Sp. Attack)
Special Defense:
Route 210 (foggy section) - Ninja Boy (upper level) - L27 Dustox (3 Sp. Defense)
Route 230 – Female Swimmer (bottom of pair) – 50 Tamanta, L52 Mantine (3 Sp. Defense)*
Route 230 – Female Swimmer (pacing, spinning, in top row of rocks) – L50 Seel, L54 Dewgong (3 Sp. Defense)*
*Not recommended because you'll have to Surf to recharge the VS Seeker in these places.
Speed:
Route 205 (water bridge section) - Fisherman (second from right) - L10, 10, 10, 10, 10, and 10 Magikarp (6 Speed)
Cycling Road - Male Triathlete (third male from top) - L37 and 39 Mukkubird (4 Speed)