If the person really believes in the shahadah - then they believe that None is worthy of worship except Allaah Alone, and Muhammad (peace be upon him) is His final messenger.
If we believe in this sincerely - then we have to fulfill the conditions which need to be met. So a person needs to keep up with their 5 daily prayers, pay zakaah etc. [i.e. the 5 pillars of islaam.]
They also have to fulfill the following conditions:
1. Knowledge: 47:19, 43:86
2. Certainty: 49:15
3. Sincerity: 39:3, 98:5
4. Truthfulness: 29:1-3, 2:8-10
5. Love: 2:165, 5:54
6. Submission: 39:54, 4:125
7. Acceptance: 25:23-25, 37:35-36
8. Al-Kufraan (Disbelieving in the Taaquut - other false deities): 2:256
If the person truely believes that there is none worthy of worship except Allaah, then they should work hard to surround their life according to the way of the final Messenger of Allaah, Muhammad (peace be upon him.)
(Say: ‘If you do love Allah follow me, Allah will love you and forgive your sins for Allah is Oft-Forgiving Most Merciful.) Qur'an (3:31)
We follow the way of the Messenger of Allaah, his sunnah. We don't follow the way of the imaams according to other sects, yes we can follow their way if their way is in accordance to the final messenger, Muhammad (peace and blessings be upon him.) And it has to be from authentic sources [i.e. Sahih Al-Bukhari, Sahih Al-Muslim etc.] We have the right to follow scholars, but at the same time we should ask them for their proofs according to how the Messenger of Allaah and his companions lived islaam.
The Messenger of Allaah (peace be upon him) said:
"Indeed those from before you from the People of the Book [christians and jews] divided into seventy-two groups. And, indeed, this group (Muslims) will divide into seventy-three. Seventy-two groups will be in the Hellfire and one of them will be in Paradise. And it is the Jamaa'ah (group)."(recorded in Ahmad)
And in another narration, he (sal Allaahu alayhi waSalam) said:
"Everyone of them in the Hellfire, except for one group that which I and my companions are upon." (recorded in Tirmidhi)
So we follow the Qur'an and Sunnah according to how the companions of the Messenger of Allaah (peace be upon him) understood it. We don't take verses/ahadith out of context, because we can only follow it according to how the Messenger of Allaah taught them. Allaah is pleased with the companions, like He says in the Qur'an for instance:
Certainly Allah was well pleased with the believers when they swore allegiance to you under the tree, and He knew what was in their hearts, so He sent down tranquillity on them and rewarded them with a near victory, [Qur'an Al-Fath 48:18]
And from among these companions were Abu Bakr, Umar, Ali, and the famous companions, according to some narrations there were 1500 believers who pledged allegiance under that tree. So if Allaah is pleased with them, they are promised paradise.
So we should follow their way because they are on the right path, on the guidance revealed to the final Messenger of Allaah, Muhammad (peace and blessings be upon him.) If we follow their way - we are successful insha'Allaah.
Allaah Almighty knows best.