To unlock the Lending a Hand trophy in Red Dead Redemption 2, you must fully complete all of the missions in the game that give Arthur a chance to be honourable.
Play honourably: you need to have Honor level 4 (average) at least in order to be able to access Chapter 6 mission "Do Not Seek Absolution". There's a set list of missions that the game defines as part of this trophy, listed below.
Money Lending and Other Sins: Activated by speaking to Herr Strauss (LS) in camp, who gives you a number of debts to collect marked by a $ on the map. Once you have collected all of the debts you can return to Strauss to finish the quest. The quest repeats over the course of Chapters 2 to 6, you need to complete them all during the chapter in which they appear:
Chapter 2: Parts I, II and III, available soon after setting up camp at Horseshoe Overlook.
Chapter 3: Part IV, available shortly after setting up camp
Chapter 4: Part V, available shortly after setting up camp
Chapter 5: N/A
Chapter 6: Part VI and VII, available shortly after setting up camp,
complete as soon as possible.Good, Honest Snake Oil: You'll find this in Chapter 2 in Valentine after your first visit to the town. Speak to the Sherrif.
Make sure to complete this before Chapter 6. The rest of the bounties do not count.
We Loved Once and True: The three parts of this mission are available in Chapter 2 and begin once you have received a letter in camp, which a camp member will inform you about if you hang around during the day after completing a few quests. Make sure you do it before accepting Dutch's final mission at the end of the chapter. Obviously choose to help the quest giver at the beginning!
The Course of True Love: You need to make sure you speak to Beau (BG) in Caliga Hall in Chapter 3 while investigating the two warring families for Dutch. After that you will need to complete two more quests for the lovers, which will appear over the coming days. Prioritise them before you finish Chapter 3.
Help a Brother Out: This is a Stranger mission in Saint Denis, that appears in the daytime opposite the Trapper in Chapter 4. Choose to help, and decline the reward.
Brothers and Sisters, One and All: You have to have completed the above mission first and wait a day. You'll find BD over by a church in north Saint Denis.
Fatherhood and Other Dreams: You'll get another letter from ML during the course of Chapter 4 which triggers this duo of missions. Make sure you do this before
Do Not Seek Absolution: You won't get this series of Chapter 6 quests unless you have at least level 4 (average) honor, and it's recommended that you have high honor anyway for the following trophy. There are two missions, make sure to check back 24-48 hours after the first.
Of Men and Angels: In Chapter 6 there is (technically) a 2-part quest from Sister Calderon, only available if you completed Brothers and Sisters, One and All in Chapter 5. The first part is just a cutscene, the second is to donate lots of food or money to the sister.
You must complete this before starting "The Fine Art of Conversation" (T) back in camp.Archaeology for Beginners: RF will appear in the Waipiti Indian Reservation during Chapter 6. When riding with Monroe, he will ask you for help. Make sure to press L2 to focus on Monroe and agree to help him for the following:
Honor, Amongst Thieves: CM will appear after the conclusion of the above mission ONLY IF you have agreed to help Monroe as above.
Mrs Sadie Adler, Widow: This one appears a fair way into Chapter 6 -- it's not the SA mission that appears very early on in the Chapter so you'll need to clear that one first. When SA appears in camp Sadie will ask for your help -- make sure to give it. Help her resolve her issues to finish the quest line.
As long as you complete all of the above missions the trophy will unlock shortly after you complete them all.